feat(timepicker): option to increment/decrement value on arrow click

I'm migrating from another timepicker and needed to do this to preserve the UX (I'm using a length of 1, where paging with arrows feels very strange). I've kept the default behavior, but this commit makes it possible to add ```data-arrows="picker"``` to the element, which changes the arrow behavior so that the model is changed instead of the displayed range of values on arrow click.

I haven't written tests or docs yet because I assume there's a possibility of discussion/tweaks before this can make it in, but I can do that if it looks good to you